1. Descartes on Certainty (and Doubt)
Michael Lacewing
© Michael Lacewing

2. Hyperbolic doubt
To find the truth, need to avoid believing error - so only believe what is ‘indubitable and certain’
What is indubitable appears to be what we are unable to doubt

3. What is certainty? Subjective, psychological feeling of conviction
Logical, e.g. a proposition that must logically be true is certain
Combined? A proposition that cannot be doubted by a rational thinker
Sometimes, we can’t doubt something because it must be true

4. Into doubt We can be deceived by our senses.
There are ‘no certain indications by which we may clearly distinguish wakefulness from sleep’.

5. The cogito
I am certain that I think, I exist. Doubting is a kind of thinking, and if I were to doubt that I existed, that would prove I do exist.
‘In this first item of knowledge there is simply a clear and distinct perception of what I am asserting’. While thinking it, I cannot doubt it.

6. ‘Clear and distinct’ ideas
Descartes comes to argue that he can know what is ‘clear and distinct’. Making this judgment requires great care, i.e. we can make mistakes.
To be clear, an idea must be ‘open and present to the attending mind’; to be distinct, it must not only be clear, but precise and separated from other ideas, so that it ‘plainly contains in itself nothing other than what is clear’ (Principles I.45).

7. Certainty
Indubitable: When I consider p carefully, I am unable not to believe it. Using my best, most careful judgment, I judge that it is impossible that it should be false: the proposition ‘is necessarily true each time it is expressed by me, or conceived in my mind’.
This is not a feeling of certainty, but a use of rational insight.

8. The wax argument
At first, our idea of the wax is of something defined by its sensory properties.
But this is muddled: when I melt a piece of wax, it loses all of its original sensory qualities, yet I believe it is the same wax.
This shows our conception of material objects, when clear and distinct, is as changeable and extended.

9. The external world: two steps to go
Meditation V: we can know that clear and distinct ideas are true; so material objects really are extended, if they exist at all.
Meditation VI: We have experiences of an external world, which must either be caused by a real external world or God. God is not a deceiver. Therefore material objects do exist.

10. The Cartesian circle
I am certain that God exists only because I am certain of whatever I clearly and distinctly perceive
And yet…
I am certain of whatever I clearly and distinctly perceive only because I am certain that God exists.

11. Descartes’ reply
I can be certain of what I clearly and distinctly perceive without knowing that God exists, but only at the time that I perceive it.
While I am clearly and distinctly perceiving some particular proposition, then I am certain of that proposition. But because of the possibility of the evil demon, I lose this certainty as soon as I turn my attention away from it, as I may be deceived that I did perceive it clearly and distinctly.

12. Descartes’ reply
God’s existence adds a general certainty that what I clearly and distinctly perceive is true.